<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title>区分undefin&null</title>
	</head>
	<body>
		<script>
			var message;
			if(message){
				alert("有值");
			}else{
				alert("无值");
			};
		</script>
	</body>
</html>
